Electrode for non-aqueous electrolyte secondary battery and production method thereof

ABSTRACT

An electrode in sheet form includes a current collector and an electrode mixture layer carried on each side thereof. The electrode is bent in the longitudinal direction thereof, to cause a large number of cracks in at least the electrode mixture layer to be positioned on the inner side of the current collector when wound, such that the cracks extend from the surface of the electrode mixture layer to the current collector in the direction intersecting with the longitudinal direction of the electrode. This bending process includes the steps of: bending the electrode at a curvature that is smaller than that of the winding core at least once; and thereafter bending the electrode at a curvature that is equal to or larger than that of the winding core. For example, this process is performed by arranging rollers such that their diameters decrease gradually and pressing the electrode against these rollers. This invention provides an electrode that does not break when wound to form an electrode assembly.

FIELD OF THE INVENTION

The present invention relates to an electrode for non-aqueouselectrolyte secondary batteries including a wound electrode assembly andto a method for producing the electrode. More particularly, the presentinvention pertains to a method for producing an electrode that does notbreak when wound.

BACKGROUND OF THE INVENTION

With the recent proliferation of portable appliances and cordlessappliances, such as cellular phones and notebook personal computers,there is an increasing demand for batteries that supply electric powerto such appliances. Among them, particularly required are secondarybatteries that are small and light-weight, have high energy density, andare capable of repeated charge/discharge.

Many of such secondary batteries include an electrode assembly composedof a positive electrode sheet, a negative electrode sheet, and aseparator interposed between the positive and negative electrode sheets,which are spirally wound together, and the wound electrode assembly isinserted in a case. Also, the capacities of such batteries are beingheightened, so attempts have been made to reduce the diameter of awinding core that is used to form a wound electrode assembly.

However, if the diameter of the winding core is reduced or the packingdensity of an electrode is increased to heighten the capacity, theelectrode may break when wound. Specifically, in the step of winding anelectrode, the electrode may break because there is a difference intensile stress between the inner and outer sides of the wound electrodedue to the thickness of the electrode.

In order to solve this problem, various proposals have been made on thestep of winding electrode sheets for forming an electrode assembly. Forexample, Japanese Laid-Open Patent Publication No. Hei 9-283152 proposespassing an electrode plate between a roll with a low surface hardnessand a roll with a high surface hardness, to cause micro-cracks in theelectrode plate in the direction intersecting with the windingdirection. This provides a flexible electrode for use in the woundelectrode assembly of alkaline storage batteries.

Also, Japanese Laid-Open Patent Publication No. Hei 11-73952 discloses atechnique applied to an electrode plate for use in cylindrical alkalinestorage batteries. According to this technique, slit lines are cut inthe part of an electrode plate to be positioned outward when wound, atgiven intervals in the direction intersecting with the windingdirection. These slit lines suppress the occurrence of cracks atirregular intervals, thereby suppressing electrode breakage uponwinding.

Japanese patent No. 3468847 discloses a technique of forming grid-likegrooves in the semidry active material paste applied to a currentcollector by using an expanded metal, in order to suppress electrodebreakage in the winding step.

The use of the technique of No. Hei 11-73952 for suppressing electrodebreakage upon winding allows an improvement in the flexibility ofelectrode plates. However, the method of sandwiching an electrode platebetween a roll with a low surface hardness such as a rubber roll and aroll with a high surface hardness allows the electrode plate to be bentonly as much as the thickness of the electrode plate at the maximum.Thus, in the winding step, this method does not allow the electrode tobe bent more than the curvature of the winding core. Also, although thismethod can cause micro-cracks in an electrode plate by giving a bendwith a small curvature to the electrode plate, such cracks are formed atlarge intervals. Thus, in the winding step, when a bending force with alarge curvature is applied to an electrode plate, a larger compressivestress is exerted on the inner part of the electrode plate. If cracksare formed at large intervals, such cracks cannot scatter thecompressive stress sufficiently. As a result, a large stress isconcentrated in one crack and the electrode plate therefore breaks uponwinding.

In the technique of No. Hei 11-73952, the slit lines in the electrodehave a depth of 5 to 10% of the electrode thickness. However, such depthis insufficient for the bending stress to be scattered or released uponwinding. Hence, deeper cracks are unevenly produced upon winding, whichmay result in electrode breakage upon winding.

Further, the application of the technique of JP 3468847 permitsformation of grid-like grooves that are deep enough to reach the currentcollector. However, since this technique uses expanded metal to formgrooves, it is difficult to form grid-like grooves at very smallintervals. Accordingly, the bending stress exerted on the electrode isnot sufficiently scattered and the stress per one crack cannot bereduced, which may result in electrode breakage upon winding.

BRIEF SUMMARY OF THE INVENTION

In view of the problems as described above, according to the presentinvention, an electrode for a non-aqueous electrolyte secondary battery,which comprises a current collector and an electrode mixture layercarried on each side of the current collector, is previously subjectedto a bending process to cause a large number of cracks in the electrodemixture layers. These cracks extend from the electrode mixture layersurface to the current collector in the direction intersecting with thelongitudinal direction of the electrode, i.e., the directionintersecting with the winding direction of the electrode. The electrodewith a large number of such cracks is wound onto a winding core togetherwith an electrode of opposite polarity and a separator, to form anelectrode assembly.

That is, a method for producing an electrode for a non-aqueouselectrolyte secondary battery according to the present inventionincludes the steps of:

forming an electrode mixture layer on each side of a current collector,to produce an electrode in sheet form; and

bending the electrode in the longitudinal direction thereof, to cause alarge number of cracks in at least the electrode mixture layer on thewinding core side such that the cracks extend from the surface of theelectrode mixture layer to the current collector in the directionintersecting with the longitudinal direction of the electrode.

In this method, the step of bending the electrode includes the steps of:

bending the electrode at a curvature that is smaller than that of thewinding core at least once; and

thereafter bending the electrode at a curvature that is equal to orlarger than that of the winding core.

The present invention is directed to a method for producing an electrodefor a non-aqueous electrolyte secondary battery, the electrode being insheet form and designed to be spirally wound onto a winding coretogether with an electrode of opposite polarity and a separator. Thismethod includes the steps of:

forming an electrode mixture layer on each side of a current collector,to produce an electrode in sheet form; and

bending the electrode in the longitudinal direction thereof bytransporting the electrode in the longitudinal direction while changingthe transport direction of the electrode under tension with a pluralityof rollers, to cause a large number of cracks in at least the electrodemixture layer on the winding core side such that the cracks extend fromthe surface of the electrode mixture layer to the current collector inthe direction intersecting with the longitudinal direction of theelectrode.

In this method, the step of bending the electrode includes the steps of:

changing the transport direction of the electrode with at least oneroller that is larger in diameter than the winding core; and

thereafter changing the transport direction of the electrode with aroller that is equal to or smaller in diameter than the winding core.

In the electrode according to the present invention, as a result of suchbending process of bending the electrode at a curvature that is equal toor larger than that of the winding core, at least the electrode mixturelayer on the winding core side has a large number of cracks that extendfrom the surface of the electrode mixture layer to the current collectorin the direction intersecting with the longitudinal direction of theelectrode. As used herein, “the electrode mixture layer on the windingcore side”, refers to the electrode mixture layer of an electrode thatis to be positioned on the winding core side of the current collectorwhen the electrode is wound onto a winding core together with anelectrode of opposite polarity and a separator to form an electrodeassembly. As used herein, “the cracks that extend from the surface ofthe electrode mixture layer to the current collector” refer to cracks ofthe electrode mixture layer that extend from the surface of theelectrode mixture layer in the direction intersecting with thelongitudinal direction of the electrode and are deep enough to reach thepart of the electrode mixture layer in contact with the currentcollector, but this does not mean that the current collector has cracks.These cracks are distributed evenly in the longitudinal direction of theelectrode. Thus, when the electrode is wound onto a winding coretogether with an electrode of opposite polarity and a separator to forman electrode assembly, the distribution of the cracks between theinitial winding position and the final winding position are basicallyalmost uniform. Accordingly, when the electrode is wound to form anelectrode assembly, new cracks do not occur and the winding stress isnot concentrated in specific portions of the electrode. As a result, inthe winding step, electrode breakage can be suppressed.

In the method of the present invention, the bending step for formingcracks that extend in the direction intersecting with the longitudinaldirection of the electrode includes the steps of bending an electrode ata curvature that is smaller than that of the winding core; andthereafter bending the electrode at a curvature that is equal to orlarger than that of the winding core. Hence, it is possible to form alarge number of cracks almost uniformly in the electrode mixture layerof the electrode. Particularly, if an electrode is bent at a curvaturesmaller than that of the winding core a plurality of times such that thecurvature increases gradually, shallow cracks produced by the firstbending gradually become deeper by the subsequent bendings. Therefore,in the step of bending the electrode, the electrode is prevented frombreaking.

The electrode of the present invention is previously subjected to abending process in which the electrode is bent at a curvature that isequal to or larger than that of the winding core. As a result, it has alarge number of cracks that extend from the surface of the electrodemixture layer to the current collector in the direction intersectingwith the longitudinal direction of the electrode. Therefore, in thewinding step for forming an electrode assembly, the electrode isprevented from breaking. Also, since the electrode mixture layer has alarge number of cracks, it has an improved ability to absorbelectrolyte, thereby making it possible to provide sufficient electrodecharacteristics.

Accordingly, it becomes possible to reduce the diameter of the windingcore to increase the energy density of the battery.

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross-sectional view of the main part of an electrode in oneembodiment of the present invention;

FIG. 2 illustrates an exemplary production step of an electrode forcausing cracks in the electrode;

FIG. 3 illustrates exemplary modified production steps of an electrodefor causing cracks in the electrode;

FIG. 4 schematically illustrates a step of winding an electrode onto awinding core that is flat in cross-section;

FIG. 5A is a schematic cross-sectional view of an electrode of thepresent invention that has been subjected to a bending process beforewinding;

FIG. 5B is a schematic cross-sectional view of the electrode of thepresent invention upon winding;

FIG. 6A is a schematic cross-sectional view of an electrode that has notbeen subjected to a bending process before winding; and

FIG. 6B is a schematic cross-sectional view of the electrode of FIG. 6Aupon winding;

DETAILED DESCRIPTION OF THE INVENTION

The electrode of the present invention is previously subjected to abending process in which the electrode is bent at a curvature that isequal to or larger than that of the winding core. Thus, of the electrodemixture layers carried on both sides of the current collector, at leastthe electrode mixture layer on the winding core side has a large numberof cracks that extend from the surface of the electrode mixture layer tothe current collector in the direction intersecting with thelongitudinal direction of the electrode.

In a preferable embodiment of the present invention, the bending processfor forming a large number of such cracks includes the steps of: bendingan electrode at a curvature that is smaller than that of the windingcore at least once; and thereafter bending the electrode at a curvaturethat is equal to or larger than that of the winding core. The step ofbending an electrode at a curvature that is smaller than that of thewinding core preferably includes pressing the electrode against a rollerthat has a diameter 50 to 200 times the thickness of the electrode.

When an electrode has a current collector thickness of 5 to 30 μm and atotal thickness of 50 to 300 μm, cracks are preferably provided atintervals of 50 to 200 μm.

In another preferable embodiment of the present invention, the bendingprocess includes the steps of: changing the transport direction of theelectrode with at least one roller that is larger in diameter than thewinding core; and thereafter changing the transport direction of theelectrode with a roller that is equal to or smaller in diameter than thewinding core.

In this case, the roller that is larger in diameter than the windingcore is a roller having a diameter 100 to 1000 times the thickness ofthe electrode.

FIG. 1 is a cross-sectional view of the main part of an electrode in oneembodiment of the present invention. An electrode 10 includes a currentcollector 11 made of metal foil and an electrode mixture layer 12carried on each side thereof. This electrode is designed to be woundonto a winding core 15, and at least the electrode mixture layer on theinner side of the current collector 11, i.e., on the winding core side,has a large number of cracks 13. The cracks 13 are formed by bending theelectrode 10 in the longitudinal direction thereof. More specifically,they are formed by bending the electrode at a curvature that is smallerthan that of the winding core 15 at least once, and then bending it at acurvature that is equal to or larger than that of the winding core 15.

FIG. 5A schematically illustrates an electrode of the present inventionthat has been subjected to a bending process before winding, and FIG. 5Bschematically illustrates the electrode of the present invention uponwinding. FIG. 6A schematically illustrates an electrode that has notbeen subjected to such a bending process before winding, and FIG. 6Bschematically illustrates the electrode upon winding. For the purpose ofsimplification, these figures illustrate only the electrode mixturelayer on the inner side of the current collector.

In an electrode 30 of the present invention, an electrode mixture layer32 has cracks 33 at regular intervals, and the cracks 33 extend from thesurface of the electrode mixture layer 32 to a current collector 31.When the electrode 30 of FIG. 5A is wound, forces are exerted as shownby the arrows of FIG. 5B. However, since this winding stress isdistributed to a large number of the cracks 33, the current collector 31does not break.

On the other hand, in an electrode 40 that has not been subjected to abending process, the winding stress is concentrated in one location asillustrated in FIG. 6B, thereby causing an electrode mixture layer 42 tohave a crack 43 that extends from the surface thereof to a currentcollector 41 and causing the current collector 41 to become cracked at alocation corresponding to the crack 43. As a result, the electrode 40breaks.

The present invention is particularly effective for positive electrodesfor use in non-aqueous electrolyte secondary batteries that are intendedto provide high energy densities. For example, LiCoO₂ has a true densityof 5.0 g/cc, and when it is used as the active material of a positiveelectrode plate, the density of the active material packed therein isapproximately 3.4 g/cc. On the other hand, LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂and LiNi_(0.85)Co_(0.10)Al_(0.05)O₂ have a true density of 4.6 g/cc, andwhen they are used as the active materials of a positive electrodeplate, the active material density is approximately 3.2 g/cc. Theseactive materials with a Ni ratio of 20% or more relative to the metalelements are difficult to pack, and if they are packed at a densitysimilar to that of LiCoO₂, the resultant electrode plate tends to becomehard. Such hard electrode plates break easily upon winding. Hence, whenthey are subjected to a bending process according to the presentinvention, remarkable effects are obtained.

The present invention is applicable not only to positive electrodes butalso negative electrodes whose electrode mixture layers are hard andpeel easily.

The present invention is preferably applied to an electrode comprising acurrent collector made of a metal foil with a thickness of 5 to 30 μmand an electrode mixture layer carried on each side thereof, theelectrode having a total thickness of 50 to 300 μm.

In the last bending step of the bending process of the presentinvention, the electrode is bent at a curvature that is equal to orslightly larger than that of the winding core onto which the electrodeis to be wound. When using rollers for the bending process, thediameters of the rollers are made equal to or slightly smaller than thatof the winding core.

The number of cracks formed in the electrode mixture layer of theelectrode is mainly determined by the first bending step under theabove-described conditions, and the depth of the cracks formed by thefirst bending step is preferably about 1/10 to ⅓ of the final depth ofthe cracks. It is desirable to perform a plurality of bending stepsbefore the last bending step such that the cracks become graduallydeeper, though the thickness of the electrode mixture layer needs to beconsidered.

It is preferred that the positive electrode plate to which the presentinvention is applied include: a positive electrode mixture comprising alithium-containing composite oxide represented by the general formulaLi_(x)M_(y)N_(1-y)O₂ (wherein M and N are at least one selected from thegroup consisting of Co, Ni, Mn, Cr, Fe, Mg, Al, and Zn, M≠N,0.98≦x≦1.10, 0≦y≦1); and a current collector, made of an Al or Al alloy,carrying the electrode mixture. The positive electrode mixturepreferably contains 0.2 to 50% by weight of a conductive agent relativeto the positive electrode active material, more preferably 0.2 to 30% byweight. When the conductive agent is carbon or graphite, the content ofthe conductive agent is preferably 0.2 to 10% by weight relative to thepositive electrode active material.

The binder to be added to the positive electrode mixture may be athermoplastic resin or a thermosetting resin. Examples includepolyethylene, polypropylene, polytetrafluoroethylene, polyvinylidenefluoride, styrene butadiene rubber,tetrafluoroethylene-hexafluoropropylene copolymer,tetrafluoroethylene-perfluoroalkylvinylether copolymer, vinylidenefluoride-hexafluoropropylene copolymer, vinylidenefluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylenecopolymer, polychlorotrifluoroethylene, vinylidenefluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylenecopolymer, ethylene-chlorotrifluoroethylene copolymer, vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidenefluoride-perfluoromethylvinylether-tetrafluoroethylene copolymer,ethylene-acrylic acid copolymer or (Na⁺) ion cross-linked materialthereof, ethylene-methacrylic acid copolymer or (Na⁺) ion cross-linkedmaterial thereof, ethylene-methyl acrylate copolymer or (Na⁺) ioncross-linked material thereof, and ethylene-methyl methacrylatecopolymer or (Na⁺) ion cross-linked material thereof. These materialsare preferably used singly or in the form of a mixture.

The negative electrode plate preferably includes: a negative electrodemixture comprising a negative electrode active material that comprises acarbon material, a graphite material, an alloy, or a metal oxide, whichis capable of charging/discharging Li; and a negative electrode currentcollector, made of Cu, Ni, or a Cu—Ni alloy, carrying the negativeelectrode mixture. Preferable binders include polyvinylidene fluoride,styrene-butadiene rubber, acrylonitrile-butadiene rubber, methylmethacrylate-butadiene rubber, methyl methacrylate-sodium methacrylaterubber, methyl methacrylate-lithium methacrylate rubber, ammoniummethacrylate-lithium methacrylate rubber, and methylmethacrylate-lithium methacrylate-ammonium methacrylate rubber. Thesematerials are preferably used singly or in the form of a mixture.

The separator is not particularly limited as long as it has acomposition capable of withstanding the operating temperature range oflithium secondary batteries. However, it is common to use a single layerof microporous film comprising an olefin resin, such as polyethylene orpolypropylene, or two or more layers thereof, and such modes are alsopreferable.

Exemplary solvents for the non-aqueous electrolyte include: cycliccarbonates such as ethylene carbonate, propylene carbonate, butylenecarbonate, and vinylene carbonate; chain carbonates such as dimethylcarbonate, diethyl carbonate, ethyl methyl carbonate, and dipropylcarbonate; aliphatic carboxylic acid esters such as methyl formate,methyl acetate, methyl propionate, and ethyl propionate; γ-lactones suchas γ-butyrolactone; chain ethers such as 1,2-dimethoxyethane,1,2-diethoxyethane, and ethoxymethoxyethane; cyclic ethers such astetrahydrofuran and 2-methyltetrahydrofuran; aprotic organic solventssuch as dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide,dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane,ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolanederivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone,3-methyl-2-oxazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ethyl ether, 1,3-propanesultone, anisole,dimethyl sulfoxide, and N-methylpyrrolidone. These are used singly or incombination of two or more of them. Among them, a mixture of a cycliccarbonate and a chain carbonte or a mixture of a cyclic carbonate, achain carbonte and an aliphatic carboxylic acid ester is preferred.

Exemplary lithium salts to be dissolved in these solvents includeLiClO₄, LiBF₄, LiPF₆, LiAlCl₄, LiSbF₆, LiSCN, LiCl, LiCF₃SO₃, LiCF₃CO₂,Li(CF₃SO₂)₂, LiAsF₆, LiN(CF₃SO₂)₂, chloroborane lithium such asLiB₁₀Cl₁₀, lithium lower aliphatic carboxylate, LiCl, LiBr, LiI, lithiumtetrachloroborate, lithium tetraphenylborate, and imides. They may beused singly or in combination of two or more of them. The inclusion ofLiPF₆ is particularly preferred.

The best mode for carrying out the invention is hereinafter describedwith reference to drawings.

FIG. 1 is a cross-sectional view of the main part of an electrode in oneembodiment of the present invention cut in the direction perpendicularto a winding core. FIG. 2 illustrates a manufacturing process forcausing cracks in an electrode.

In one embodiment of the present invention, an electrode 10 is woundaround a winding core 15. The electrode 10 includes a current collector11 and an electrode mixture layer 12 carried on each side thereof, andat least the electrode mixture layer on the inner side of the currentcollector 11 has a large number of micro-cracks 13. These cracks of theelectrode mixture layer preferably extend to the current collector 11.

A method for producing the electrode 10 with such cracks is describedwith reference to FIG. 2.

The electrode 10 has been previously wound around a roller 21. Theelectrode 10 unwound from the roller 21 is bent with a roller 22, atension roller 23, and rollers 24, 25, 26, 27, and 28, and thentransported to a section 29 for forming a spiral electrode assembly. Dueto the winding force of the winding core 15 in the section 29, theelectrode 10 is transported from the roller 21 to the roller 28. Theaxes of the respective rollers are parallel, and the axis of the tensionroller 23 is movable upward and downward in this figure, whereby thetension exerted on the electrode is adjusted.

The diameters of the rollers gradually decrease toward the section 29.For example, the diameter of the roller 22 is 50 mm, the diameters ofthe rollers 23 and 24 are 30 mm, and the diameters of the rollers 25,26, 27, and 28 are 15, 10, 5, and 4.5 mm, respectively. The diameter ofthe winding core 15 is 4.5 mm. It should be noted that these figures areintended only for illustration, and that the relative sizes of therespective elements are not necessarily correct. For example, thethickness of the electrode 10 illustrated therein is larger than theactual size, as compared with the roller 22, for example. Also, thediameter of the roller 21, from which the electrode is unwound, issignificantly larger than that of, for example, the roller 22, and theelectrode mixture layers of the electrode have no cracks when beingwound around the roller 21.

By adjusting the position of the tension roller 23, an appropriatetension is applied to the electrode 10, and the electrode 10 istransported under tension as shown by the arrows of the figure. Theelectrode 10 first comes into contact with the roller 22 and is stronglypressed against about ¼ of the circumference of the roller 22. That is,the transport direction of the electrode is changed 900 by the roller22. As a result, a large number of micro-cracks occur in the electrodemixture layer of the electrode 10. Although it depends on the rollerdiameter, the cracks formed by the first roller 22 will gradually becomedeeper by the subsequent rollers and ultimately extend to the currentcollector. After being passed though the tension roller 23, theelectrode 10 is pressed against the rollers 24 to 27, whose diametersgradually decrease, about ½ the circumference of each roller. Lastly,the electrode 10 is pressed against the roller 28 having the samediameter as that of the winding core 15, ¼ the circumference thereof. Inthis way, desired cracks are formed in the electrode mixtures layer ofthe electrode. It should be noted that although the diameters of therollers 24 to 27 gradually decrease, they may be the same. That is, inthe case of using n rollers, the diameter r_(n-1) of the n−1^(th) rollerand the diameter r_(n) of the n^(th) roller satisfy the relation:r_(n)≦r_(n-1).

The tension exerted on both ends of the part of the electrode in contactwith the roller is preferably 20 to 200 gf/cm, though it varies with thestrength of the current collector of the electrode. When the width of anelectrode to be bent is 5 cm, the tension is preferably 100 to 1000 gf.

The positions of the cracks in the electrode mixture layer are mostlydetermined when the electrode is pressed against the first roller 22,and the depths of the cracks are gradually increased by the subsequentrollers whose diameters gradually decrease. Therefore, the diameter ofthe first roller 22 and the degree of contact of the electrode with therollers are important. When the electrode has a large thickness, thediameter of the first roller 22 needs to be increased, so that thenumber of the subsequent rollers is also increased.

For example, suppose that the diameter of the roller 22 is 1000 timesthe thickness of the electrode, for example, the diameter is 100 mm whenthe thickness of the electrode is 100 μm. In this case, when thediameter of the roller 22 is twice or more than twice the diameter ofthe winding core, the subsequent rollers up to the roller 28, which hasa diameter that is equal to or smaller than that of the winding core,desirably have diameters that gradually decrease by about 50%. Also,when the diameter of the first roller is less than twice the diameter ofthe winding core, the subsequent rollers up to the last roller 28 havediameters that gradually decrease by about 50%.

When the thickness of an electrode to be wound onto a winding core of2.5 to 5 mm in diameter is 0.05 to 0.30 mm, it is preferred to causecracks in the electrode mixture layer that is to be positioned inwardwhen wound at intervals of 0.05 to 0.2 mm. In order to cause suchcracks, it is preferred that the diameter of the roller 22 be 15 to 100mm and that the electrode be pressed against the roller 22 at least ¼the circumference thereof.

In FIG. 2, the tension roller 23 and four rollers 24 to 27 are providedbetween the roller 22 and the roller 28, and the electrode is pressedagainst each roller about ½ the circumference thereof. This is intendedto gradually increase the depths of the cracks caused by the firstroller 22 and, at the same time, to evenly cause cracks in the electrodemixture layers on both sides of the current collector by bending theelectrode in the winding direction and the direction opposite to thewinding direction. Also, by alternately bending the electrode in thewinding direction and the opposite direction, the electrode can betransported smoothly and worked in a desired manner without requiring along transport space. Thus, the number of the rollers may be selected asappropriate, depending on the thickness of the electrode mixture layersof the electrode, the diameter of the winding core, etc.

In FIG. 2, the diameter of the last roller 28 is the same as that of thewinding core 15, but the diameter of the roller 28 may be slightlysmaller than that of the winding core. The bending direction by the lastroller 28 is preferably the same as the winding direction of theelectrode.

While FIG. 2 shows a preferable embodiment, FIG. 3 shows a modifiedembodiment.

FIG. 3( a) illustrates an exemplary first bending step of an electrode10A. The transport direction of the electrode 10A is changed at an angleθ by a roller 22A. Even if the angle θ is a very small value, forexample, about 1°, it can cause cracks in the electrode 10A.

In this case, the roller 22A does not necessarily have a larger diameterthan that of the winding core. The roller 22A causes almost even cracksin the electrode mixture layer by changing the transport direction ofthe electrode. However, in the case of bending with a roller that has asmaller diameter than that of the winding core, if the angle at whichthe transport direction is changed is increased, the curvature of thebending becomes greater than that of the winding core. Therefore, in thecase of bending with a roller having a smaller diameter, it is desirablethat an electrode be partially brought into contact with thecircumference of the roller.

FIG. 3( b) shows an exemplary bending step of the electrode 10A, wherean auxiliary roller 28B is provided to more effectively bend theelectrode 10A with a last roller 28A, and the electrode 10A is pressedagainst the roller 28A about ⅔ the circumference thereof. In the case ofbending an electrode with the last roller of a plurality of rollers, theelectrode is preferably pressed against the last roller at least ¼ thecircumference thereof, as illustrated in FIG. 2. Specifically, it ispreferred that the transport direction be at 90° or more. In FIG. 3( b),the electrode is pressed against the roller 28A approximately ⅔ thecircumference thereof. It should be noted that the auxiliary roller 28Bis designed to change the direction of the electrode 10A and that itdoes not contribute to the bending of the electrode 10A.

FIG. 2 has shown an instance in which the winding core is cylindricaland the electrode assembly is accommodated in a cylindrical batterycase. However, the present invention is also applicable to an instancein which an electrode assembly fabricated by using a winding core thatis flat and rectangular in cross-section is accommodated in arectangular battery case. FIG. 4 schematically illustrates a step ofwinding an electrode 10B onto a winding core 15B that is flat andrectangular in cross-section. The winding core 15B has a slit 16 in theaxial direction thereof, and the edge of the electrode 10B is fittedinto the slit 16 and wound onto the winding core 15B. When such awinding core is used, the diameter of the core is defined as the shorterside r_(B).

Examples of the present invention are hereinafter described.

Example 1 Preparation of Positive Electrode Plate

100 parts by weight of lithium composite oxide LiCoO₂ was mixed with 4parts by weight of acetylene black, serving as a conductive agent, andan N-methyl-2-pyrrolidone (hereinafter referred to as NMP) solutioncontaining 4 parts by weight (solid content) of polyvinylidene fluoride(PVdF #1320, available from Kureha Chemical Industry Co., Ltd.), servingas a binder, and the mixture was kneaded to form a paste of electrodemixture. This paste was applied to both sides of a current collectormade of a 15-μm-thick aluminum foil, dried at 110° C. to evaporate NMP,and rolled to a thickness of 200 μm. The true density of the LiCoO₂ was5.0 g/cc (tap density 3.0 g/cc), and the density of the resultantelectrode mixture layer was 3.6 g/cc. Subsequently, this was cut to awidth of 50 mm, to prepare a positive electrode plate.

Thereafter, this positive electrode plate was subjected to a bendingprocess.

This bending process was performed by using the apparatus as illustratedin FIG. 2. The diameters of the respective rollers are the same as thoseas described above with reference to FIG. 2. As a result, the electrodemixture layer to be positioned inward when wound had cracks at intervalsof 90 to 100 mm, and these cracks extended from the surface of theelectrode mixture layer to the current collector.

(Preparation of Negative Electrode Plate)

100 parts by weight of flake graphite, serving as the active material,was mixed with an aqueous dispersion containing 3 parts by weight (solidcontent) of styrene-butadiene rubber, serving as a binder, and 3 partsby weight of a sodium salt of carboxymethyl cellulose, serving as athickener, to form a paste of electrode mixture. This paste was appliedto both sides of a current collector made of a 15-μm-thick copper foil,dried at 110° C. to evaporate the water, and rolled to a thickness of200 μm. Subsequently, this was cut to a width of 52 mm to prepare anegative electrode plate.

(Inspection for Cracks)

The positive electrode plate subjected to the bending process was cut inparallel with the winding direction and wound onto a core having thesame diameter as that of a winding core that will be described later. Inthis state, the depths of the cracks in the electrode mixture layer ofthe electrode were measured. In order to facilitate the measurement ofthe crack depth, the electrode was colored with lead ions, and thecolored electrode was subjected to an energy-dispersive elementalanalysis for lead mapping. The detected depth of lead was defined ascrack depth.

(Production Method of Wound Electrode Assembly)

A spiral electrode assembly was produced by winding the 50-mm-widepositive electrode plate subjected to the bending process, the52-mm-wide negative electrode plate not subjected to the bendingprocess, and a separator for separating these two plates around awinding core of 4.5 mm in diameter such that the positive electrodeplate was positioned inward. The separator was a 20-μm-thickpolyethylene porous film (available form Asahi Kasei Corporation). Theporosity of the separator measured by a mercury porosimeter (availablefrom Yuasa Ionics Inc.) was 45%. When the positive electrode plate, thenegative electrode plate, and the separator were wound together to formthe electrode assembly, a tension roller was disposed between each hoopand the winding location at which they were wound such that a tension of200 gf was exerted on each of them. FIG. 2 shows the tension roller 23,which applies tension to the positive electrode.

(Inspection Method of Electrode Used in Electrode Assembly for Breakage)

The wound electrode assembly was unwound to separate the positiveelectrode plate, the negative electrode plate, and the separator. Thepositive electrode plate subjected to the bending process was examinedfor its breakage, the length of splits, and the like, to confirm theeffect of the present invention in suppressing electrode breakage.

(Production Method of Battery)

A wound electrode assembly, which was prepared in the same manner as theelectrode assembly subjected to the above-mentioned inspection, wasplaced in a stainless steel SUS battery case, and an electrolyte wasinjected therein. The opening of the battery case was sealed with asealing plate with a positive electrode terminal and an insulatinggasket.

The resultant cylindrical lithium secondary battery had a height of 65mm, a diameter of 18 mm, and a design capacity of 1800 mAh. Thenon-aqueous electrolyte was prepared by dissolving LiPF₆ at 1 mol/l in asolvent mixture of ethylene carbonate and ethyl methyl carbonate in avolume ratio of 1:1.

Comparative Example 1

In the apparatus of FIG. 2, a positive electrode was transported fromthe roller 24 to the section 29 without being passed through the rollers25 to 28. A battery was produced under the same conditions as those ofExample 1 except for the use of this positive electrode.

Comparative Example 2

In the apparatus of FIG. 2, a positive electrode was transported fromthe roller 26 to the section 29 without being passed through the rollers27 and 28. A battery was produced under the same conditions as those ofExample 1 except for the use of this positive electrode.

Comparative Example 3

In the apparatus of FIG. 2, a positive electrode was pressed against ¼of the circumference of the roller 28 as a bending process without beingpassed through the rollers 22 to 27. A battery was produced under thesame conditions as those of Example 1 except for the use of thispositive electrode.

Example 2

A positive electrode with an electrode mixture density of 3.2 g/cc wasproduced by using LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ with a true density of4.6 g/cc (tap density 2.8 g/cc) as the positive electrode activematerial. This positive electrode was subjected to the same bendingprocess as that of Example 1.

Example 3

A positive electrode with an electrode mixture density of 3.2 g/cc wasproduced by using LiNi_(0.85)Co_(0.10)Al_(0.05)O₂ with a true density of4.6 g/cc (tap density 2.8 g/cc) as the positive electrode activematerial. This positive electrode was subjected to the same bendingprocess as that of Example 1.

Examples 4 to 14

Positive electrodes with an electrode mixture density of 3.3 g/cc wereprepared by using the positive electrode active material mixtures ofExamples 1 to 3 as shown in Table 1. These positive electrodes weresubjected to the same bending process as that of Example 1.

TABLE 1 Mixing ratio of active material A:B:C Bending process Example 11:0:0 Example 2 0:1:0 Same as Example 1 Example 3 0:0:1 Same as Example1 Example 4 1:5:0 Same as Example 1 Example 5 1:3:0 Same as Example 1Example 6 1:1:0 Same as Example 1 Example 7 3:1:0 Same as Example 1Example 8 5:1:0 Same as Example 1 Example 9 1:0:5 Same as Example 1Example 10 1:0:3 Same as Example 1 Example 11 1:0:1 Same as Example 1Example 12 3:0:1 Same as Example 1 Example 13 5:0:1 Same as Example 1Example 14 1:1:1 Same as Example 1 Example 15 1:0:0

In Table 1 and Table 2, “A” represents LiCoO₂, “B” representsLiNi_(1/3)Mn_(1/3)Co_(1/3)O₂, and “C” representsLiNi_(0.85)Co_(0.10)Al_(0.05)O₂.

Example 15

A positive electrode was produced in the same manner as in Example 1,except that the diameter of the roller 28 of the apparatus of FIG. 2 waschanged to 3.5 mm. This positive electrode was subjected to the samebending process as that of Example 1.

Comparative Examples 4 to 20

These comparative examples are the same as Example 1, except for the useof positive electrode active materials and positive electrodes subjectedto bending processes as shown in Table 2.

TABLE 2 Mixing ratio of active material A:B:C Bending processComparative Example 1 1:0:0 Comparative Example 2 1:0:0 ComparativeExample 3 1:0:0 Comparative Example 4 0:1:0 Same as Comparative Example1 Comparative Example 5 0:1:0 Same as Comparative Example 2 ComparativeExample 6 0:1:0 Same as Comparative Example 3 Comparative Example 70:0:1 Same as Comparative Example 1 Comparative Example 8 0:0:1 Same asComparative Example 2 Comparative Example 9 0:0:1 Same as ComparativeExample 3 Comparative Example 10 1:5:0 Same as Comparative Example 1Comparative Example 11 1:3:0 Same as Comparative Example 1 ComparativeExample 12 1:1:0 Same as Comparative Example 1 Comparative Example 133:1:0 Same as Comparative Example 1 Comparative Example 14 5:1:0 Same asComparative Example 1 Comparative Example 15 1:0:5 Same as ComparativeExample 1 Comparative Example 16 1:0:3 Same as Comparative Example 1Comparative Example 17 1:0:1 Same as Comparative Example 1 ComparativeExample 18 3:0:1 Same as Comparative Example 1 Comparative Example 195:0:1 Same as Comparative Example 1 Comparative Example 20 1:1:1 Same asComparative Example 1

Each of the batteries thus produced was charged at a constant voltage of4.2 V (maximum current 1 A) in an environment at 25° C. for 30 minutesand then discharged at a constant current of 0.2 A down to a cut-offvoltage of 3.0 V. In this way, the initial capacity was obtained. Also,each battery was subjected to repeated cycling of charging at a constantvoltage of 4.2 V (maximum current 1 A) in an environment at 25° C. for30 minutes and then discharging at a constant current of 1 A down to acut-off voltage of 3.0 V. In this way, the cycle characteristic(capacity retention rate relative to initial capacity) was obtained.

Also, positive electrode plates subjected to the bending process wereexamined for their ability to absorb electrolyte, by immersing them inan electrolyte for 1 minute and measuring their weight increase oneminute after pulling them up. The electrodes used for this comparison ofelectrolyte absorption had a width of 30 mm and a length of 200 mm.

These results are shown in Table 3 together with the depths of crackscaused by the bending process and the state of electrode breakage whichwere inspected in the above manner. In Comparative Examples 10 to 20,their electrodes completely broke in the direction of the width duringthe winding step and could not be used to assemble batteries. The amountof electrolyte absorbed was 1.0 to 1.3 g.

TABLE 3 Capacity Amount of Initial retention electrolyte Capacity rateat Crack depth absorbed (mAh) 500th cycle (%) (%) Breakage (g) Example 11990 85 100 Not broken 2.3 Example 2 1990 87 100 Not broken 2.0 Example3 2000 87 100 Not broken 2.0 Example 4 1990 83 100 Not broken 2.1Example 5 1990 85 100 Not broken 2.2 Example 6 1990 86 100 Not broken2.2 Example 7 1990 83 100 Not broken 2.2 Example 8 1990 85 100 Notbroken 2.2 Example 9 2000 84 100 Not broken 2.1 Example 10 2000 83 100Not broken 2.2 Example 11 1995 86 100 Not broken 2.2 Example 12 1995 87100 Not broken 2.2 Example 13 1990 86 100 Not broken 2.2 Example 14 199086 100 Not broken 2.2 Example 15 1990 93 100 Not broken 2.3Comparative * — — Completely 1.2 Example 1 broken in width directionComparative 1990 Capacity  70 50% broken in 2.4 Example 2 lowered to 1%width direction during cycling Comparative * — Current 50% broken in 1.3Example 3 collector width direction broken Comparative * — — Completely1.3 Example 4 broken in width direction Comparative 1990 Capacity  6040% broken in 2.3 Example 5 lowered to 1% width direction during cyclingComparative * — Current 25% broken in 1.3 Example 6 collector widthdirection broken Comparative * — — Completely 1.3 Example 7 broken inwidth direction Comparative 1990 Capacity  60 50% broken in 2.2 Example8 lowered to 1% width direction during cycling Comparative * — — 25%broken in 1.3 Example 9 width direction * Due to electrode breakageduring winding, a battery could not be assembled.

As in Example 1, the electrodes subjected to the bending processaccording to the present invention were free from breakage in thewinding step and exhibited desired battery characteristics. Anobservation of a cross-section of each electrode before the winding stepshowed that the cracks of the electrode mixture layer were deep enoughto reach the current collector surface. It was thus confirmed that thesecracks of the electrode mixture layer that were deep enough to reach thecurrent collector surface scattered the bending stress exerted on theelectrode in the winding step, thereby making it possible to suppressthe breakage of the electrode. It was also confirmed that the method ofthe present invention enables formation of desired cracks in electrodes.

On the other hand, in Comparative Example 1 where the bending process ofthe present invention was not performed, no cracks with a sufficientdepth were observed in a cross-section of the electrode. Also, when theelectrode assembly was unwound to measure the size of electrodebreakage, the electrode was found to be broken at a positionapproximately 8 mm from the end close to the winding core.

The electrodes subjected to the bending process were compared in termsof the ability to absorb electrolyte, and the comparison showed that theelectrode of Example 1 has a large ability to absorb electrolyte. Thiscan heighten the permeation speed of the electrolyte into the electrode,thereby increasing the amount of the electrolyte held by the electrodeand improving the cycle characteristics. Also, this can help reduce thetime of the electrolyte injection step.

Also, in Example 15 where in the last stage of the bending process, theelectrode was bent with the roller of 3.5 mm in diameter, which wassmaller than the 4.5-mm-diameter of the winding core, the currentcollector was not damaged in the bending process and the breakage of theelectrode was suppressed in the winding step.

On the other hand, in Comparative Example 2 where in the last stage ofthe bending process, the diameter of the roller was larger than that ofthe winding core, the bending process was insufficient. Hence, thecracks of the electrode mixture layer were not deep enough to extend tothe current collector surface, and partial breakage occurred in thewidth direction. In the battery including such an electrode, theelectrode was broken at this breakage due to the impact of electrodeexpansion/contraction during charge/discharge, so that the batterycapacity lowered significantly during the evaluation of cyclecharacteristics.

In Comparative Example 3 where the bending process was performed withonly one roller whose diameter was equal to that of the winding core, anobservation of a cross-section of the electrode revealed that during thebending process with the roller, the current collector was alreadypartially broken.

In Examples 2 to 14, LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ orLiNi_(0.85)Co_(0.10)Al_(0.05)O₂, which has a lower true or tap densityand a lower packing rate of active material than LiCoO₂ was used as thepositive electrode active material, and the bending process wasperformed in the same manner as in Example 1. In these examples, thecracks of the electrode mixture layer were deep enough to reach thecurrent collector surface, and these cracks allowed the bending stressto be scattered sufficiently upon winding. On the other hand, inComparative Examples 2 to 20, their electrodes were completely orpartially broken in the winding step, since they did not have suchpreferable cracks as those of the present invention.

Example 16

A positive electrode and a negative electrode were produced in the samemanner as in Example 1, but the positive electrode had a width of 42 mmand the negative electrode had a width of 43 mm. The positive electrodewas subjected to a bending process in the same manner as in Example 1 byusing the apparatus of FIG. 2, except that the diameter of the lastroller was changed to 4.5 mm. The positive and negative electrodes thusproduced and a separator were wound together onto a flat winding corehaving a longer side of 25 mm and a shorter side of 4.5 mm to form anelectrode assembly, and a rectangular battery of 50×34×5.2 mm wasproduced. This battery had an initial capacity of 920 mAh and a capacityretention rate at the 500th cycle of 95%. Meanwhile, a positiveelectrode produced in the same manner as the above was subjected to abending process in the same manner as in Comparative Example 1, and thenwound with a negative electrode and a separator. However, during thewinding, this positive electrode completely broke in the direction ofthe width and could not be used to assemble a battery.

As described above, according to the present invention, no electrodebreakage occurs in the winding step for forming an electrode assembly.Therefore, the diameter of a winding core can be reduced to increase theenergy density of the battery. The present invention is particularlyapplicable to high-energy-density non-aqueous electrolyte secondarybatteries for use in portable appliances and cordless appliances.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

1. An electrode for a non-aqueous electrolyte secondary battery,comprising: a current collector; and an electrode mixture layer carriedon each side of said current collector, said electrode being in sheetform and designed to be spirally wound onto a winding core together withan electrode of opposite polarity and a separator, wherein at least theelectrode mixture layer on the winding core side has a large number ofcracks that extend from the surface of the electrode mixture layer tothe current collector in the direction intersecting with thelongitudinal direction of the electrode. 2-4. (canceled)
 5. Anon-aqueous electrolyte secondary battery comprising an electrodeassembly, said electrode assembly comprising a positive electrode, anegative electrode, and a separator interposed between the positiveelectrode and the negative electrode, which are wound onto a windingcore, wherein at least one of the positive electrode and the negativeelectrode is the electrode of claim 1.